Research On In-Situ Remediation Of Nitrate Pollution In Groundwater By Microbial Fuel Cell

Microbial fuel cell technology (MFC) is an emerging environmental technology for energyrecovery from the wastewater. Early studies of MFC mainly focused on how to improve itsperformance of electricity production. The further research indicated that the source of MFC fuelvaried from the easy degradation substance,such as glucose, sodium acetate to refractory organicsubstance,such as phenol, pyridine, etc. Thus an idea was provided that the MFC could be used inthe field of dealing with contaminated wastewater. But present researches were mostly limited tosmall pilot scale in the water-soluble medium, and the conclusions were not suitable for groundwaterin porous media system.Nitrate pollution in groundwater is becoming increasingly serious presently. In-situ remediationtechnologies are better among its treatment technologies,such as in-situ bioremediation technologiesand the permeable reactive barrier(PRB). Based on this, the idea is to design MFC devices used tosimulate the in-situ remediation of nitrate contamination in groundwater and the MFC’s performanceof bioremediation and electricity generation in porous media was also investigated. Two differentstructure MFCs were designed for in-situ remediation of nitrate pollution of groundwater, one was asingle-chamber MFC and the other was a dual-chamber MFC. Some factors affecting the MFCperformance of the removal of nitrogen and carbon and electricity production were investigatedsystematicly, including the COD concentration in the groundwater, MFC’s external resistance valueand groundwater pH environment.The main results in the thesis are summarized as follows:1.Two large volume MFC devices were construced according to the practical nitrate pollution ingroundwater.The titanium-based lead dioxide electrode was used as the cathode and two carboncloths buried in sand were usd as the anode in the single-chamber MFC device. In the dual-chamberMFC device, cathode and anode material was carbon cloth and the salt bridge was used to connectthe anode chamber and cathode chamber.Before operation,some hydrodynamic parameters of themodel device were determined. The results showed that the two devices (especially thesingle-chamber MFC device) were effective when used to simulate the processing on the nitratepollution in groundwater, and this laid the foundation for MFC’s actual application in the in-situremediation of nitrate pollution in the groundwater.2.The self-made titanium-based lead dioxide electrode had been used in MFC for a better effecton treating nitrate wastewater in water-soluble media system.A variety of electrochemical methodswere used to study the electrode catalytic properties in this work.The cyclic voltammograms(CV) ofthe electrode shown that the electrode self was not involved in denitrification in the MFC, itincreased the nitrogen removal and decontamination of the MFC because of its higher current response. And the electrochemical impedance spectroscopy(EIS) shown that the electrode’selectrochemical activity in the MFC reaction system was enhanced after a period of reaction time,which explained the phenomenon mechanicaly that a period of set-up time was requried in order toobtain a stable voltage output from MFC device.3.The results of single-chamber MFC device showed that it could take advantage of theorganics in the wastewater as fuel (corresponding to the COD concentration of1000mg/L and500mg/L)and obtain a stable voltage output with simultaneous removal of organic matter and nitratewhen treating the wastewater containing nitrate concentration of100mg/L. And the best operatingparameters were obtained during the period of in-situ remediation of nitrate pollution in groundwater.The reults were as follows: the COD concentration was500mg/L and the pH value was7.0ingroundwater, MFC external resistor value was800. Under this condition, the MFC’s performanceof decontamination and electricity generation was quite good as follows: removal of carbon—CODconcentration in the ultimate overflow tank outlet was212mg/L with the removal efficiency of57.6%(maximum COD removal efficiency of76.2%, part time during the experiment); remomal ofnitrate—Nitrate nitrogen value was very small during the whole period with the maximum value ofonly5.15mg/L, and nitrite nitrogen was not accumulated too. But the ammonia nitrogenconcentration was quite high due to nitrate dissimilation and the NH4Cl addition into the buffersolution; electricity production—During the entire experiment period, the external output voltagewas quite stable.The highest voltage was413mv and the value was still maintained at about300mvat the end of the experiment, which showed the device’s continuious and stable voltage outputperformance.The maximum power density was56.36mW m-2with the corresponding current densityof187.68mA m-2. All the results indicated that the single-chamber MFC device obtained highremoval of nitrogen and carbon with effective power output.4.The results of dual-chamber MFC devices running in the same parameters withsingle-chamber MFC devices’ showed that both the performance of decontamination and theelectricity production were poor when dealing with the wastewater containing nitrate concentrationof100mg/L. This may be due to the the short length of the device operation time partialy, but it alsodemonstrated that the design and construction of the device were required further improvement. Thetime and space variation of the nitrate concentration in the two devices was investigated. for the nextstep to establish the solute transport model in groundwater within the experimental sand tank. Andthis would lay the foundation for theoretical study on the migration of nitrate in groundwater anddetermination of nitrate transport in porous media system.5.The electrochemical workstation was used to analyse the electrode’s electrochemical behaviorin the solution obtained from the two kinds of MFC devices. The scanning electron microscopy (SEM) was used to observe the surface morphology of the activated sludge biofilm attached to thecarbon cloth electrode. The technology of polymerase chain reaction (PCR) and denaturinggradient gel electrophoresis (DGGE) were used to study the bacteria community distribution of thebiofilm. Electrochemical studies had shown that the lead dioxide electrode(used in thesingle-chamber MFC) electrochemical activity would decrease in the long run, but the anode carboncloth electrode(used in the dual-chamber MFC)electrochemical activity would increase in thesolution obtained at the end of the experiment. A thick layer of biofilm could be observed on thesingle chamber MFC carbon cloth surface, and dual-chamber anode carbon cloth was covered with athick layer of rod-shaped bacteria biofilm, but few bacteria were seen on the dual-chamber cathodecarbon cloth. The domiant species of microbial community composition of the single-chambercarbon cloth biofilm were Fluviicola taffensis, Flavobacteriales bacterium, MoraxellaceaeBacterium, Acinetobacter sp, Flavobacterium sp, Janthinobacterium lividum, Comamonadaceaebacterium; while the domiant species of microbial community composition of the dual-chambercarbon cloth biofilm were Hyphomicrobium sp, Rhodobacter sp, Thermomonas fusca, Thermomonasfusca, Hirschia baltica, Arcobacter sp and Rhodobacter.The results show that in-situ remediation of nitrate pollution in groundwater by MFC deviceis theoretically possible. As long as the good performance of the electrode material and reasonabledesign of the device, MFC technology will play an important role in the engineering practice of.in-situ remediation of groundwater pollution.